Piezoelectric material can convert mechanical energy into electrical energy or electrical energy into mechanical energy. A typical example of the piezoelectric material is lead zirconate titanate (Pb(Zr, Ti)O3, hereinafter referred to as PZT) having a perovskite crystal structure. PZT has an extremely excellent piezoelectric property, and has been conventionally used in many electronic components. Also currently, PZT is used as one of main piezoelectric material.
Examples of piezoelectric devices using piezoelectric material such as PZT include a piezoelectric actuator, a piezoelectric sensor, and an inkjet head for a printer. In recent years, in order to carry out microprocessing thereof, technology using MEMS (Micro Electro Mechanical Systems) has been developed actively.
For applying the piezoelectric device using PZT to MEMS, a film of PZT needs to be formed on a substrate as a diaphragm. Examples of a method of forming a film include a sputtering method, a sol-gel method, a MOCVD method, and the like. When such methods are used, piezoelectric films having high quality can be obtained and high-performance piezoelectric property can be expected. On the other hand, in these methods, a film formation speed is slow, and thus, efficient productivity of piezoelectric elements that require a film thickness of 1 μm or more cannot be obtained. Furthermore, as the substrate to be the diaphragm, from the viewpoint of low cost and easy processing, studies using stainless steel substrates are carried out actively.
Recently, as a method of forming a piezoelectric film on such a substrate, an aerosol deposition method (hereinafter, abbreviated as an AD method) has been investigated. The AD method is based on a principle that fine particles are integrated with each other by thermal energy generated by allowing the particles to collide with a substrate at a subsonic speed. With this method, since a piezoelectric film can be formed at a high speed, and, furthermore, the film can be formed with high adhesion because it is anchored to an upper surface layer of the substrate and, further, film can be highly dense.
Patent Literature 1 discloses a piezoelectric film-laminated structure in which PZT having a film thickness of 10 μm or more is formed on a stainless steel substrate by an aerosol deposition method (AD method). This piezoelectric film-laminated structure is formed by forming a diffusion layer by diffusing additive elements contained in the metal substrate onto the metal substrate by heat treatment. Therefore, in this piezoelectric film-laminated structure, adhesion strength is higher and free from peeling or crack as compared with conventional structures.
However, employment of the AD method poses many problems. For example, it is necessary to transport submicron powders by high-pressure air, thus making maintenance difficult. In addition, a vacuum system is required at the time of film formation. From such reasons, the AD method has not been popularized so well.
Furthermore, a piezoelectric layer can be formed by a printing method. The printing method has obtained much attention as “printable electronics,” that is, technology for printing electronic components. Furthermore, the printing method does not require a complicated device. Whit this method, film formation time is extremely short even when a film thickness is 10 μm or more, and a necessary amount may be formed to a necessary portion.
However, when a piezoelectric layer is formed in simple equipment by a printing method that is excellent in mass productivity, the piezoelectric layer usually includes 40% to 50% per volume of voids. Therefore, it is necessary to carry out firing at a high temperature so as to increase density. At the time of firing, since a piezoelectric layer is restrained by a substrate, the piezoelectric layer does not contract in the plane direction, but contracts only in the thickness direction of the piezoelectric layer. Therefore, the piezoelectric layer may be peeled off from the substrate due to tensile and compression stress generated between the piezoelectric layer and the substrate. Furthermore, the piezoelectric layer may be peeled off from the substrate by combination of residual stress remaining after firing and vibration occurring when a piezoelectric element is driven.